The wrong screw can quietly eat a hole through an aluminum gutter

When two different metals touch in the presence of moisture, they can form a tiny electrochemical cell — the same basic reaction as a battery. One metal acts as the anode and slowly sacrifices itself to the other. Aluminum gutters are especially prone to this: fasten one with an incompatible screw, and the aluminum around that contact point becomes the anode and corrodes first, often showing up as pitting or a small leak right at the screw.

Counterintuitively, a plain stainless steel screw can be worse for aluminum than a zinc-galvanized one, because the zinc coating on galvanized steel is less reactive against aluminum, while bare stainless steel is not. Near the coast, salt-laden air acts as the electrolyte that makes the reaction run faster, so this shows up sooner on homes closer to the water.